1. Field of the Invention
The present invention relates to a charge pump circuit, and more especially, to a charge-pump circuit using a reverse current cut-off circuit to improve the electronic efficiency and reliability.
2. Background of the Related Art
In recent years, such a charge pump circuit has been used frequently as a power supply circuit capable of outputting a voltage higher than an input voltage without using an inductor, and supplying a power supply voltage to a load requiring a relatively small consumption current. FIG. 1 shows a circuit diagram of conventional dual phase charge pump circuit. The charge pump circuit includes a diode-connected NMOS transistor and number of capacitors. The diode-connected NMOS transistor has a threshold voltage and connects to a node where two NMOS transistors couples with. The circuit's operation is that the circuit has two inverse impulse signals φ1 and φ2 to charge capacitors for the purpose of raising voltage. According to the conductive path the circuit's clock cycle has a setup period and a pumping period. In the setup period, the pulse signal φ1 is low level, and the dual phase charge pump circuit 100 turns on the first charging path 110, then the NMOS transistor is turned on for raising the voltage of first node 120 to Vin−Vt, and the voltage of capacitor C1 is charged to Vin−Vt. In the pumping period, the pulse signal φ1 is high level to Vφ, and the voltage of capacitor C1 is at same level Vφ to raise the voltage of first node 120 to Vin+(Vφ−Vt). When the first node's 120 voltage is higher than the threshold voltage Vt in the NMOS transistor, the second charging path 130 is closed to raise the voltage of the second node 130 to Vin+(Vφ−Vt)−Vt. Reasonably, at next period the voltage of the second node 130 is going to Vin+2(Vφ−Vt).
To summarize, a dual phase charge pump circuit with n nodes has the output voltage shown in equation (1-1)Vout=Vin+N*(Vφ−Vt)−Vt  (1-1)
From equation 1-1, because the effect of the MOS's characteristic of threshold voltage, the charging efficiency in the dual phase charge pump circuit 100 is decreased, thus causing inefficiency for the system. Especially in low power supply condition, the threshold voltage has great effect on the operation of the dual phase charge pump circuit. In some applications, the inefficiency of the dual phase charge pump circuit will consume more power when charging the circuit, and is their shortcomings.
Many varieties of charge pump circuit were released to overcome the problem, for example the FIG. 2 shows an improved charge pump circuit 200 disclosed in U.S. Pat. No. 6,670,844. The charge pump circuit 200 includes depletion-type N-channel MOS transistors each having its gate and drain interconnected via a diode connection and individually having their respective sources and drains interconnected thereby defining multiple stages interconnected in cascade, and capacitor elements individually connected to the respective sources of the MOS transistors. The charge pump circuit 200 operates as follows. A given input voltage is applied to the drain and gate of the initial stage MOS transistor1. Then a clock signal and an inverted clock signal are alternately supplied to the MOS transistors via the respective capacitor elements so that a boosted voltage is obtained from the final stage MOS transistor. But at the switching of clock cycle, the characteristic of depletion-type MOS transistors cannot make the charge pump circuit 200 turned off as the voltage of node A is raised. Then a reverse current leakage Iq, which is equal to the effect of charging inefficiency caused by the threshold voltage of enhancement-type NMOS transistor in FIG. 1, decreases the system's efficiency. Furthermore, the overflow from reverse current leakage Iq sometimes damages the system and lowers the reliability; so the system need extra protect circuit to keep the reliability. In contrast, this causes the product's cost and disadvantages.
For reducing the reverse current leakage Iq, the improved charge pump circuit 200 changes the W/L ratio of the depletion-type NMOS transistors. But this way just minimizes the reverse current leakage Iq, and cannot stop it all. Besides the difficulties of manufacturing will increase the cost and the electronic interference, and make the reliability decreased.
For this reason, a charge pump circuit with the stable reliability and without reverse current in needed.